Without limiting the scope of the invention, its background is described in connection with sigma delta modulators, as an example.
Heretofore, in this field, sigma delta modulators are used in analog-to-digital converters. Analog-to-digital converters have seen increased use in recent years due to the advances in digital signal processing and the increased use of digital transmission systems. Typically, analog-to-digital converters comprise circuitry for receiving an analog input signal and outputting a digital value that is proportional to the input analog signal. One type of analog-to-digital converter is one that utilizes sigma-delta modulation.
The sigma-delta modulator converts an analog input into a digital pulse string having an average amplitude over time proportional to the analog input. Sigma-delta modulation provides for high accuracy and wide dynamic range, as compared to the earlier delta modulation techniques. The sigma-delta type modulation is sometimes referred to as an oversampled converter architecture which is immune from some of the earlier undesirable second order effects of delta modulation. The sigma-delta modulator is an oversampling type in which the analog input voltage is sampled at a rate that is substantially higher than the desired output sampling rate of the converter.
Sigma-delta modulation employs feedback of the output pulse train, or reference voltages or currents controlled by the output pulse train, to a summing node to which the input signal is also supplied. The difference between the two signals is then integrated. The integrator typically is an operational amplifier which has a capacitor as the feedback element. The integrated signal is compared to a predetermined reference level. When the reference level is crossed, a gate is opened or closed, either beginning or terminating an output pulse and simultaneously changing the signal being fed back to the summing node. The integrated signal is thus usually maintained in the vicinity of the reference level and, assuming an input signal varying sufficiently slower than the output pulses, the output pulse train will carry information reflecting the input amplitude to a high degree of accuracy.
Typically, if the output of a sigma-delta integrator goes into saturation (or approaches saturation), the circuit will be immediately reset to a specified value by a hard reset. The hard reset sets the output immediately to a specific value regardless of what the output value was immediately before the reset. When this hard reset occurs, relevant information will be lost in the process. Also, after a hard reset, there is a delay before the system returns to its normal operating condition.